The importance of the Ras small GTPases in normal cell physiology and the aberrant behavior of cancer cells is well-established. Ras functions'as a signaling node that regulates normal cell proliferation, differentiation and survival. The diverse cellular roles of Ras are mediated through Ras regulation of multiple, functionally distinct, downstream signaling networks, with the Raf-MEK-ERK mitogen-activated protein kinase (MARK) cascade the best understood. The mutational activation of Ras results in persistent, deregulated signaling that promotes the aberrant growth and behavior of malignant cancer cells. Consequently, there is considerable interest and effort in targeting Ras signaling for the development of novel approaches for cancer treatment. In particular, current evaluation is focused on clinical evaluation of small molecule kinase inhibitors of the Raf-MEK-ERK pathway. However, these efforts have been complicated by the fact that Ras can also utilize multiple Raf-independent effector pathways to promote oncogenesis. Furthermore, the Raf-MEK-ERK cascade is not a simple linear signaling pathway, but instead, represents the core of a complex signaling network that is regulated by an ever-expanding roster of functionally diverse signaling molecules. In particular, several scaffold proteins dictate the input signals that activate this kinase cascade and serve to diversify the spatial and temporal nature of the signaling output. Our recent preliminary observations determined that (a) the subfamily-D regulators of G protein signaling (RGS) proteins (RGS12 and RGS14) are putative effectors of Ras and that (b) RGS12 may function as a scaffold protein that regulates the Raf-MEK-ERK cascade. In light of the important role of the ERK MARK pathway in neoplastic cell biology, we hypothesize that RGS12 and RGS14 will be found to be important effectors of Ras-mediated oncogenesis and that RGS12 will be a critical regulator of Raf-MEK-ERK signaling and function. Four specific aims are proposed to critically evaluate the role of subfamily D RGS proteins in aberrant Ras and Raf signaling and oncogenesis to: (1) determine the roles of RGS12 and RGS14 as effectors of Ras-mediated oncogenes, (2) determine if RGS12 is critical for mutant B-Raf-mediated oncogenesis, (3) determine if RGS12 is an endosome-specific effector of Ras function, and (4) determine the structures of RGS12 (and/or RGS14) with Ras-MAPK binding partners.